1. Field of Invention
This invention relates to the field of target assemblies for use with accelerators for the production of radioisotopes. More particularly, this invention pertains to target assemblies, which have less than ideal thermal conductivity, having internal cooling channels and thermally optimized target chambers.
2. Description of the Related Art
Positron Emission Tomography (PET) is a powerful tool for diagnosing and treatment planning of many diseases wherein radioisotopes are injected into a patient to diagnose and assess the disease. Accelerators are used to produce the radioisotopes used in PET. Generally, an accelerator produces radioisotopes by accelerating a particle beam and bombarding a target material, housed in a target system, with the particle beam.
Several factors must be considered when developing a target system for the production of radioisotopes. In the case of gas or liquid targets, the target material must be maintained at an elevated pressure during bombardment to compensate for the effects of density reduction of the target material due to heating/expansion/phase change (boiling). Further, it is desirable to operate at higher beam currents to increase production of the radioisotopes. Because of the amount of heat generated during bombardment, cooling the target material and other components of the target system is of significant importance.
Enriched water targets are used for the commercial production of the short lived (t1/2=109.8 minutes) positron emitter fluorine-18 (18F) for use as a tracer for Positron Emission Tomography (PET). The desired isotope is produced by proton bombardment of 18O enriched water (enrichment typically above 95%), using the 18O(p,n)18F reaction. The 18F isotope is used to produce fluorodexyglucose (FDG), which, when introduced within a patient, is used to map metabolic rates in the patient.
The cost of the enriched water and the short half-life of 18F drive competing constraints on the target design. In order to overcome decay losses the target production must be maximized. This requires the target assemblies be designed for maximum operating current, which also increases ionization heating of the bombarded water. In order to minimize cost of reagents (specifically the expensive enriched water), the target assemblies necessarily have a small volume (<2 ml). Typical volume averaged power density in such targets is 400 W/cc. However peak power densities can be as much as two orders of magnitude greater.
FIGS. 1 and 2 illustrate perspective views of a prior art target assembly 110 showing the front surface 112 and rear surface 212, respectively. FIG. 3 is a cross-sectional view of the target assembly 110. The target assembly 110 has a front face 112, which is adapted to connect to an accelerator or cyclotron. The target assembly 110 has a cylindrical body which fits into a cylindrical slot which supplies cooling water to the target assembly 110. The target assembly 110 also has a rear face 212, which has connections 220, 222 for the enriched water and openings for securing 214, 216 the target assembly 110.
The prior art target assembly 110 includes a target chamber 104 encased in silver and having cooling channels 102, 102′, 202, 204, 302, 304 along the outside surface of the target assembly 110. Typically, cooling water flows into the channel 102′ on the bottom of the target assembly 110, through the channels 302, 304 along the circumference of the target assembly 110 and the channels 202, 204 along the rear surface 212 of the target assembly 110, and collecting in the channel 102 on the top of the target assembly 110, where it is removed and run through a heat exchanger to remove the collected heat. The channels 302, 304 are formed between the fins 306, 308 positioned around the circumference of the target assembly 110. In the illustrated prior art target assembly 110, the first fin 306 is separated from the other fins 308 by a larger gap, or channel, 302 in order to allow the target assembly 110 to receive a fastener.
The prior art target assembly 110 includes a target chamber 104, which is filled with enriched water via an inlet port 220 on the back side 212. The target chamber 104 is sealed with a window 310 adjacent the front face 112. The inlet port 220 feeds an inlet channel 106, through which the enriched water enters and fills the target chamber 104. The air pushed out of the target chamber 104 exhausts through the outlet port 222. Before being irradiated, the enriched water completely fills the target chamber 104.
The prior art target assembly 110 is fabricated from a silver ingot and operates at approximately 600 watts (10 MeV protons at 60 μA) on the target water. Irradiation of 18O-water in silver target bodies with proton beam currents higher than 30 μA generally leads to formation of gray or black colloids which frequently clogs the 18F ion delivery lines. More importantly, the reactivity of the 18F ion thus obtained is severely diminished. A model of the prior art target assembly 110 has been generated. This model of the external coolant cycle exposed inefficient cooling mechanisms, opportunities for coolant dryout, and likelihood of flow instabilities.
Silver target assemblies 110 oxidize under the conditions seen in a high pressure water target, and eventually this oxidation leads to failure of the system, both through increased temperature drops through the oxide, sequestering of the fluoride product on the oxide surface, and oxide particles fouling the product capillary tubing and subsequent synthesis into the desired tracer. At high currents, such as 40-60 μA, the silver target holders are typically only usable for 20 to 30 runs to create radioisotopes such as Flourine-18 before being too contaminated for further use to maintain sufficiently pure radiochemicals. At that point the target assembly must be removed from the accelerator and cleaned to recover functionality.
Various factors effect the production of radioisotopes from liquid targets with low energy accelerators. One such factor includes the configuration of the holding assemblies that retain the liquid target during the irradiation process. The holding assemblies must withstand severe environments created during the irradiation process and also enable the production of contaminant-free radiochemicals. When the liquid target is irradiated, the proton beam quickly heats the liquid target and creates high pressure within the target holder. The target holder must be capable of withstanding the elevated pressures without rupturing and without removing too much energy from the proton beam. Conventional liquid target holders have a thin front window through which the proton beams must pass before hitting the liquid target. Thicker windows are desirable to withstand the pressures generated from heating the liquid, but the thicker windows provide more mass through which the proton beam must pass before reaching the target. Accordingly, the thicker windows absorb more beam energy, thereby decreasing the effectiveness of the proton beam. When a low energy beam is used, it is highly desirable to ensure that as much energy remains in the proton beam as possible by the time it reaches its liquid target to maximize the beam's efficiency for irradiating the liquid target. So, while the strength of the thick window is desired, the resulting energy decrease in the beam is not.
Another factor includes providing a liquid target that will fully absorb the remaining energy of the proton beam. As the proton beam is passed into the target holder and the target liquid, the target liquid must have a sufficient depth or thickness so as to fully absorb the particles from the beam. If the proton beam passed completely through the liquid target and the target holder, the particle beam could create a radioactive environment external to the holding assembly.
Another significant factor in forming the radioisotopes or radiochemicals is controlling the target liquid's temperature during the irradiation process. When the proton beam bombards the target liquid, the temperature of the target liquid quickly increases. Heat must be efficiently drawn from the target liquid to maximize the effective density of the target liquid.
The quantity of radioisotopes produced in a liquid target is very small (e.g., an isotope concentration in the target may be in the order of 10−12), so it is important that the target body not introduce contaminants into the target material. Such contaminants would reduce the quantity of the available useful radioisotopes, and hinder the subsequent chemical processes in incorporating the radioisotope into the desired radiochemical.
Removal of the heat generated in the target is a significant problem that limits the magnitude of the incoming beam's current and hence, the production rate. Higher production rates are achieved if beams with higher currents can be used. Prior art target holders have been made of silver, which has a high thermal conductivity that allows heat to be quickly drawn from the liquid target. The silver target holders, however, often introduce impurities such as silver oxides that can react with or impede the reaction of the radiochemical formed in the target holder.
A description of water targets is provided in an article titled “Tantalum [18O] Water Target for the Production of [18F] Fluoride with High Reactivity for the Preparation of 2-Deoxy-2-[18F]Fluoro-D-Glucose,” by N. Satyamurthy, Bernard Amarasekera, C. William Alvord, Jorge R. Barrio, Michael E. Phelps, in Molecular Imaging and Biology, Vol. 4, No. 1, at 65-70 (2002). This article describes the use of tantalum for the body of the water target and discloses some of the disadvantages and problems of the prior art silver target assemblies. The article further discloses the lower heat conductivity of tantalum, along with its chemical inertness, radiochemical reactivity, and low induced activation. FIG. 1 of the article illustrates that the target assembly is cooled by heat transfer into a cooling water plenum located inside the assembly. Test results using tantalum show an average actual yield of 112.7 mCi/μA for the nine runs over 60 minutes in duration. This yield is 68.3% of the theoretical yield. None of the documented tests had a beam current above 40 μA and the beam energy was at 10.8 MeV.
An example of target cooling is disclosed in U.S. Pat. No. 5,917,874, titled “Accelerator Target,” issued to Schlyer, et al. on Jun. 29, 1999, which discloses a target 14 with radial cooling fins 28. The Target 14 contains a sample 12 in the front side and a cooling system on the back side. The cooling system includes an integral solid cone 42 with a grouping of radial fins 28 disposed on the outer surface of the cone 42 to increase the surface area for cooling. A water jet 40a is directed at the apex 42a of the cone 42 from a single center inlet 40d. The coolant 40a flows along the cone 42 and radial fins 28, through a plenum 40c, and out a pair of outlets 40e. 
U.S. Pat. No. 6,586,747, titled “Particle Accelerator Assembly With Liquid-Target Holder,” issued to Erdman on Jul. 1, 2003, discloses a target assembly 12 with two windows 62, 64. The target cavity 60 has a front window 62, formed of Havar, through which the particle beam 17 passes. The target cavity 60 has a thin rear window 64, formed of a thin section of the holder body 56, formed of niobium, which separates the target cavity 60 from the cooling channel 74. Transfer of the heat from the target cavity 60 is through the rear window 64 and by passing cooling fluid through the cooling block 68 and over the rear window 64. The cooling block 68 is mounted to the holder body 56 and has support ribs 72 that form parallel cooling channels 74 through which the cooling fluid flows. The target cavity 60 is at an angle to the particle beam 17, thereby allowing the particle beam 17 to pass through a greater thickness of the target fluid 54, which allows for using higher energy particle beams 17.